In the train of increased environmental awareness and of the effort to produce products in an ever more efficient way, solventless coating operations are becoming more and more established within the pressure-sensitive adhesives (PSAs) segment.
A solventless coating operation, however, imposes exacting requirements on the PSAs to be processed, since their viscoelastic properties govern not only the processability but also their later profile of properties. There are often contradictions between the requirements imposed on the viscoelasticity of the PSA that are favorable for processing and for the product properties. For example, products with a high shear strength frequently comprise polymers having high degrees of polymerization. This, however, leads to high melt viscosities and hence to more complicated and expensive solventless processability.
A PSA applied by solventless coating frequently acquires its ultimate structure only after a crosslinking step, which sets the balance between adhesive and cohesive properties in the product. By way of example, mention may be made here of chemical crosslinking operations, such as the vulcanization of rubbers, for example, of radiation crosslinking operations, such as the UV treatment of polyacrylates, for example, and of physical crosslinking operations, such as the thermoreversible formation of high-softening-point domains in synthetic rubber-based systems.
Polyacrylates-based PSAs are one of the most important basic types of adhesives in self-adhesive tapes, being employed in particular in challenging and high-value applications. Qualities that are utilized with these products are the high weathering and UV stability and also the accessibility of embodiments having a water-clear transparency, in conjunction with an eminently settable balance between bond strength and thermal shear strength. Furthermore, there is a wide range of monomers for constructing the PSAs, leading to further property control possibilities. PSAs based on polyacrylates are also not immune from the trend toward solventlessly coatable systems.
Examples of solventlessly coatable polyacrylates used as PSAs can be found in the literature. A distinction is made here between two concepts: the use of polymers having an initially low degree of polymerization, which following the coating operation are crosslinked by exposure to actinic radiation; and the use of what are called acrylate hotmelts, consisting of polymers with a high melt viscosity at room temperature. Systems belonging to the first group are very amenable to coating, but the cohesive properties of the product, as manifested, for example, in the thermal shear strength, are often capable of improvement. Examples are disclosed in EP 377 199 by BASF and U.S. Pat. No. 4,181,752 by 3M. Approaches belonging to the second group are described for example in U.S. Pat. No. 5,391,406 from National Starch, WO 93/09152 by Avery Dennison or DE 195 242 50 by Beiersdorf. Although good product properties are found for the systems they describe, the solventless coating of these PSAs is often difficult.
DE 100 29 554 describes a process for producing crosslinkable acrylate PSAs with molar masses of more than 200 000 g/mol. In a first step it prepares polyacrylates from a monomer mixture which in one embodiment may comprise acrylic and methacrylic acid monomers and one or more carboxylic anhydrides having olefinic double bonds. In a second step the resultant polyacrylate composition is admixed with further monomers, which possess at least two functional groups. The first functional group is intended to react with the carboxylic anhydride, while the second functional group is a crosslinkable group. Examples of monomers of this kind are acrylates containing hydroxyl groups. The PSAs obtained can subsequently be blended with crosslinkers. Suitable crosslinkers are difunctional or polyfunctional acrylates, difunctional or polyfunctional isocyanates or difunctional or polyfunctional epoxides. The acrylate PSAs are intended for processing from the melt.
DE 100 30 217 likewise discloses a polyacrylate PSA which comprises polymers with molar masses of more than 250 000 g/mol and can be blended with a crosslinker, such as with a difunctional or polyfunctional acrylate, difunctional or polyfunctional isocyanate or difunctional or polyfunctional epoxide, for example. The polyacrylate PSA is applied from a solution to the backing material.
DE 41 27 513 discloses binder mixtures for coating materials which consist of a polyisocyanate component and a hydroxy-functionalized polyacrylate component. The two components are mixed with one another in a solvent and the mixture is applied to a backing material. DE 42 20 807 likewise describes the addition of crosslinkers to a vinyl polymer in a solvent.
DE 43 24 801 discloses a process for producing a coating material. To a first, polymeric component it adds a second component, which can comprise a crosslinker. The first component may constitute (meth)acrylic copolymers containing OH groups, while polyisocyanates are among the proposed second-component candidates. The addition of the second component to the first component is said to be possible without solvent provided the components have a low viscosity. The systems described, however, are unsuitable for PSA utility. Molar masses of the polymers described are situated at not more than 8600 g/mol.
DE 103 59 973 describes a process for producing PSAs which are based on acrylate block copolymers. The acrylate block copolymer is obtained by linking two chemically different acrylate polymer blocks. DE 101 57 695 discloses a process for preparing polymers by linking two components. The first component is a poly(meth)acrylate having a number-average molecular weight of 2000 to 100 000 g/mol, with functional groups formed on its ends. The second component likewise has functional groups at its ends. Linking of the two components takes place by the functional groups located at the ends of the components. For the preparation of the first component, special measures are needed in order to realize the end-functionalization.
None of these citations, accordingly, discloses a process by which easily accessible components can be reacted solventlessly from the melt.
There is therefore a need for a PSA system, and a coating operation for this PSA system, to be provided, so that the appropriate twinning of PSA system and coating operation resolves the disadvantages of the prior art, namely the improvable combination of good solventless-coating properties and good product properties, particularly in respect of thermal shear strength.